1. Field of the Invention
The present invention relates to a heat-developable photosensitive material capable of forming an image by a dry process and to an image forming method which uses the heat-developable photosensitive material.
2. Description of the Prior Art
A silver salt photography method, which uses a silver halide, is a recording method which is used widely because of its excellent sensitivity and gradation expression capability. However, the fact that the process after exposing the image is performed in a wet manner causes problems to occur such that the material is unsatisfactory to work with, unsatisfactory to handle and unsatisfactory from a safety standpoint.
In the aforesaid circumstance, dry material not developed by a wet process has been researched and the results have been disclosed in Japanese Patent Publication No. 43-4921 and Japanese Patent Publication No. 43-4924. The aforesaid technology employs photosensitive silver halide in amounts sufficient to form a silver latent image and non-photosensitive organic silver salt as the image forming material. The organic silver salt serves as the image forming material because: (1) a latent image is formed by imagewise exposure on the photosensitive silver halide; and (2) the latent image serves as a catalyst when the photosensitive material is heated, which causes the organic silver salt and the reducing agent to take part in an oxidation-reduction reaction to reduce the organic silver salt to silver which forms the image.
Heat-developable photosensitive material has been used in a variety of industrial photosensitive materials in the image communication field, the medical field and computer output field because of its advantage that an image can be formed by using a dry process in place of the wet process.
Since the heat-developable material provides excellent photosensitivity and can easily be sensitized in the visible region because it contains silver halide as the photosensitive element, it has been used in a recording system which includes a light source comprising a gas laser, such as a He-Ne laser, an Ar ion laser and the like, which has an oscillation wavelength in the visible light region.
In recent years, a semiconductor laser has been developed. The cost, the size and the weight of the laser is much reduced. The semiconductor laser provides efficient output as compared with the gas laser and has been put into practical use in an optical disc such as a compact disc and a laser printer. Hence, it is expected that a dry image recording system, the cost and the size of which will be reduced and which provides excellent performance, will be realized by using a laser light source, and a heat-developable photosensitive material.
In order to improve photosensitivity with respect to long wavelength light and, in particular, to red light, a cyanine dye has been used in the conventional silver halide photosensitive material for the gelatin-type dry process. However, the cyanine dye has been considered to be unsuitable because it has a poor sensitizing efficiency with respect to the heat-developable photosensitive material for the dry process.
A merocyanine dye for use as the sensitizing dye to be contained in the heat-developable photosensitive material and disclosed in Japanese Patent Publication No. 49-18808 has encountered an unsolved problem of unsatisfactory stability of the sensitizing dye and in the sensitizing efficiency.